Optimizing oil well operations has been a challenge since the drilling of the first oil well. When wells produce oil over some time period, reservoir pressures decline and some form of artificial lift (pumps, gas lift, etc.) is required to lift reservoir liquids accumulating in the subsurface well bore to the surface. In the United States, the predominant form of artificial lift is the rod pumping system whereby a string of rods connected to a subsurface pump moves the plunger of the subsurface pump in a reciprocating motion to lift well fluids accumulating in the well bore.
The three principle problems encountered in the management of a rod pumping system are as follows:
(1) Adjusting the capacity of the pump to the ability of the reservoir to deliver fluids from the reservoir to the well bore.
(2) The identification of inefficiencies in the well pumping system to effect remedial efforts before such inefficiencies result in excessive cost and/or lost production. PA1 (3) The identification of reservoir problems that restrict the flow of fluids from the reservoir to the well bore so that remedies may be implemented to restore reservoir performance when such remedies are economical. PA1 (1) Changing reservoir pressures due to depletion of fluids from the reservoir or the injection of fluids from external sources into the reservoir or both. PA1 (2) Changing well bore permeabilities because of deposition of foreign materials in pore spaces of the reservoir. PA1 (3) changing reservoir fluid characteristics that restrict or enhance the flow of fluids from the reservoir to the well bore. PA1 (1) Liquid displacement measurement will allow the operator to observe what effect, if any, the adjustable idle-time of the pump (to allow liquid accumulation in the annulus) has on the deliverability of the reservoir. With conventional pump-off controllers, the effect of adjusting the adjustable idle-time on reservoir deliverability must be measured using conventional well test facilities which is expensive and time consuming. PA1 (2) Liquid displacement measurement will allow the operator to observe long term and small changes in pump displacement due to worn components (plungers, barrels, balls and seats) such that remedies to replace worn components can be immediately employed when such remedies are economically feasible. PA1 (3) Liquid displacement measurement will permit pump-up time measurement. (Pump-up time is the time interval from when the pump is again started ater being shut off for liquids to accumulate in the annulus until liquids first appear at the surface flow lines.) Changes in pump-up time are representative of changes in pump efficiency and provide an indication of the necessity or economics of pump or well repair. This measurement is not possible with conventional "pump-off" controllers. PA1 (4) Liquid displacement measurement will allow observation or indication of change and magnitude of change of reservoir deliverability of the individual well. This is extremely important information in optimizing daily production (producing the maximum amount of oil at minimum cost each day) and in evaluating secondary or enhanced recovery efforts by correlating individual well and reservoir fluid injection rates, volumes, patterns, and pressures with that of the individual well and reservoir withdrawal rates. PA1 (5) Liquid displacement measurement will permit automatic control of pump speed (strokes per minute) or control of both pump speed and time pumped to provide a further improvement in the performance of the lift system to increase reservoir deliverability of the individual well and decrease lift costs. Control of pump speed with conventional pump-off controllers is difficult if not impossible since the variable (pump speed) directly affects rod loading. PA1 (1) They are bulky, require considerable capital expenditure, and are difficult to maintain at individual well sites. PA1 (2) Failure modes of the valving and instrumentation can result in the restriction of flow from the well. PA1 (3) Controllers and instrumentation result in significant pressure differential between the inlet and outlet of the separation vessel resulting in some loss of reservoir deliverability. PA1 (1) The subject invention to measure liquid in a fluid stream containing both liquids and gases is relatively small, inexpensive, and simple compared to conventional liquid measurement systems in the same environment. PA1 (2) The subject invention to measure well head liquids will not obstruct or restrict flow of well fluids from the inlet to the outlet of the apparatus in any failure mode of the valving or instrumentation of the apparatus. This feature assures any well operator of no loss of production or resultant hazardous conditions due to failures.
If both reservoir deliverabilities and pumping capacity were constant, if would probably be feasible to manually adjust pumping capacity to equal that of reservoir deliverability. In practice, neither is constant. Reservoir deliverability changes because of many factors some of which are as follows:
Pump capacities change because of wear on component parts--pump plunger, barrel, balls and seats, or other components. Pump capacities may also change because of the deposition of material (scale/paraffin) in or around the pump barrel so as to restrict the entry of well fluids from the well bore into the pump barrel on the upstroke of the pump.
Present efforts to adjust rod pumping capacities to reservoir deliverabilities has, for the most part, consisted of controlling the percentage of time the pumping system operates over some specific time period. Usually, the percentage of time that a pumping well operates is manually adjusted with a percentage or interval timer. Because the well operator wishes to be sure that the well produces all the oil or revenue that the reservoir is capable of delivering, he will adjust the time the well pumps slightly in excess of reservoir deliverability. This excess pumping capacity, to assure maximum production, results in both an increased amount of electrical power required to lift the oil and increased wear and tear on the pumping system over that required under ideal conditions. Also, since both the pump capacity and reservoir deliverability are continually changing, frequent testing or monitoring of the well and the lift system is required to be sure that the well is producing at maximum rates and the lift system is operating near maximum efficiency.
In practice, keeping the well properly adjusted by manual means is difficult if not impossible. In most cases, if the capacity of a pump is within plus or minus ten percent of reservoir deliverability operators feel that the well is operating within optimum conditions. In reality, such practice results in excess costs or lost production. To alleviate problems caused by over or underpumping, "pump-off" controllers were developed to automatically adjust pumping time to match reservoir deliverability. These controllers provided some means to detecting a "pumped-off" condition and then shutting off the pump for some fixed but adjustable interval to allow liquids to again accumulate in the well bore. "Pumped-off" conditions are defined as those conditions in the pumping resulting in decreased subsurface pump displacement. This condition is usually caused when liquid levels in the well bore or annulus fall to levels at or near the subsurface pump inlet. Such conditions will cause incomplete filling of the pump barrel on the upstroke of the pump which results in decreased pump displacement. This decreased pump displacement is caused by pumping at rates in excess of reservoir deliverability. When a well pumps in excess of reservoir deliverability, liquids which have accumulated in the well bore or annulus when the well was not pumping will eventually fall to a point at or near the subsurface pump inlet. At some point when annulus levels are near the pump inlet, liquids will fail to completely fill the pump barrel on the upstroke of the pump causing a decrease in volumetric displacement.
Liquid levels in the annulus at which incomplete filling or reduced displacement occurs will vary depending on many factors such as pump design, pump speed, reservoir fluid characteristics, or restrictions to flow between the well bore (annulus) and the pump barrel.
The majority of "pump-off" controllers developed and in operation detect pump-off (reduced displacement) by monitoring directly or indirectly changes in rod loading during periods of reducing displacements as liquid levels fall in the annulus of the well. These units employ load sensors on the rods or on the beam of the pump system or they use current or speed sensors to detect changed loading conditions of the drive motor caused by changing rod loads when liquids fall to fill the pump barrel. When displacement changes are detected, the well is shut down for some fixed but adjustable time interval to allow some liquids to accumulate in the annulus before the pumping system is again started. There have been several documented case histories where these conventional "pump-off" controllers have resulted in significant improvement in operations. Electrical and maintenance costs have been reduced. In some cases production gains have been realized where wells were underpumped by previous manual timing methods or where excessive well down time was experienced because of excessive maintenance due to overpumping. In spite of these documented successes, there are still many more manually adjusted time controlled wells than those automatically adjusted by conventional "pump-off" controllers. Much of the reluctance to employ "pump-off" controllers to automatically adjust the cycling of the wells is due to the difficulties in the maintenance, adjustment, and care required for the conventional pump-off controllers.
The subject invention (hereafter called pump optimizer) overcomes many of the disadvantages of and has many desirable features not available in present conventional "pump-off" controllers which utilize changing rod loading to indirectly detect change in pump displacement. The pump optimizer directly measures pump liquid displacement with excellent repeatability by accurately determining the volume of liquid pumped by a few up and down strokes of the pump unit. Any change in pump displacement (due to incomplete or decreased filling of the pump barrel on the upstroke) is quickly and easily detected by the pump optimizer for operator information and action or automatic control of pump cycling whichever mode of operation is desirable.
Directly measuring pump displacement (liquid quantity pumped each stroke of the pump) has several desirable features; some of which are as follows: